Vehicle hydraulic dual brake systems have included pressure control valves for reducing the potential for premature rear wheel lock during braking. These pressure control valves generally limit the rear wheel brake pressure relative to the front wheel brake pressure in order to reduce the rear wheel brake torque and thus prevent premature rear wheel lock. A drawback of the type of pressure control valve which is used in a diagonal split brake system is that, in the event of failure in one branch of the brake system, the pressure control valve continues to limit the rear wheel brake pressure in the unfailed branch of the brake system thereby reducing the available remaining rear wheel brake torque that is then needed for emergency braking. Therefore, it is an object of this invention to provide a pressure control valve for diagonal split brake systems which limits the rear wheel brake pressure relative to the front wheel brake pressure during normal braking but which precludes the limiting of the rear wheel brake pressure in the event of partial brake system failure so that the rear wheel brake which is disposed in the unfailed branch of the brake system can develop the maximum brake torque for emergency braking.